In an existing protocol, an RFSP (RAT/Frequency of Selection Priority, where RAT stands for Radio Access Technology) index parameter has been introduced to support radio resource management in an RAN (Radio Access Network), and an RFSP index can be divided into a subscribed RFSP index and an RFSP index in use. The subscribed RFSP index is retrieved from an HSS (Home Subscriber Server), and the RFSP index in use, i.e., an RFSP index which is being used, is generated by an MME (Mobility Management Entity) according to a configuration policy of a visited network. The MME will store both of the RFSP indexes, and in a non-roaming status where the RFSP index in use is the same as the subscribed RFSP index, the MME will provide the RAN with the RFSP index-in-use parameter, and the RAN can obtain a cell reselection priority of a UE (User Equipment) from this RFSP index parameter to thereby control an access of the UE in an idle mode and also possibly redirect the UE in an active mode onto another RAT.
In the existing protocol, the RFSP index is configured statically according to an policy of a local operator and subscription information of a user and will not be modified by the MME, and it can be transported from the MME to the RAN in an Initial Context Setup Request message and a UE Context Modification Request message. The UE Context Modification Request message is intended to modify an set-up UE context and unavailable when UE is in the idle mode, and at present, when UE is in the idle mode, there are the following conditions to trigger the MME to transmit the Initial Context Setup Request message:
1. There is uplink data at the UE, and then an active flag can be carried in a TAU Request (TAU stands for Tracking Area Update) message to notify the MME to initiate a user plane bearer setup procedure via S1 interface;
2. The MME is notified of downlink data coming; and
3. A Service Request procedure.
At present a Long Term Evolution (LTE) network primarily provides a PS (Packet Switched) domain service while its voice service is based upon an IMS (IP Multimedia System), but some UE may not support an IMS voice capability or they can not select an IMS voice service due to a configuration policy of an operator, and at this time the network has to provide the UE with voice service of a CS (Circuit Switch) domain, thus necessitating a CS Fallback (Circuit Switched Fallback (typically CSFB or CS Fallback for brevity)) enabling the UE to access a 2G/3G network. However the CSFB may result in a specific delay, so in order to optimize an experience of a user, an enhanced RFSP index function has been proposed in the document S2-096004 so that the MME can modify and then transport the RFSP index in use to the RAN which in turn instruct the voice service-centric UE to reselect a cell and to be attached to the 2G/3G network without performing any CSFB. As further described in 23.401 of the document, the MME can generate a new RFSP index in use according to the subscribed RFSP index, voice capability and setting information provided by the UE and the configuration policy of the local operator and transport the new RFSP index in use to the RAN in a message via the S1 interface. Reference can be made to 23.401-S2-096004-4.3.6: Radio Resource Management functions for details.
A drawback of the prior art lies in that it has been proposed in the prior art only that after the capability and setting of the UE has been changed, the MME can modify and notify the RFSP index to the RAN in the Initial Context Setup Request message via the S1 interface, and then the RAN can use the RFSP index to select an access of the UE.
However the new RFSP index in use can not be transported until the UE triggers the MME to transmit the Initial Context Setup Request message.